Fixes #4085: Really get all machine inventories for list of node displaying

The problem was that we only requested machine inventories in the "accepted" branch. But sometimes, when nodes are accepted/removed/accepted, machine inventories end in the "deleted" branch. So we need to query all inventories. 
For that to not be to costly, I restrained the returned LDAP entries' attributes to only "objectClass", since it's the only one needed to decide if the machine is virtual or not.